sedimentological and palaeohydrological...

Sedimentological and palaeohydrological responses to tectonicsand climate in a small, closed, lacustrine system:Oligocene As Pontes Basin (Spain)

A. SAEZ and L. CABRERADepartment of Stratigraphy, Palaeontology and Marine Geosciences, Research Group of Geodynamicsand Basin Analysis, Faculty of Geology, University of Barcelona, E-08028 Barcelona, Spain(E-mail: [email protected])

ABSTRACT

A small, closed, lacustrine system developed during the restraining overstep

stages of the Oligocene As Pontes strike-slip basin (Spain). The increase in

basin accommodation and the headward spread of the drainage, which

increased the water input, triggered a change from shallow, holomictic to

deeper, meromictic conditions. The lower, shallow, lacustrine assemblage

consists of mudstone–carbonate cycles recording lacustrine–palustrine ramp

deposition in a saline lake. High Sr content in some early diagenetic calcites

suggests that aragonite and calcite made up the primary carbonate muds. Early

dolomitization took place together with widespread pedogenic activity. The

upper, deep, freshwater, lacustrine assemblage includes bundles of carbonate–

clay rhythmites and fine-grained turbidite beds. Primary calcite and diagenetic

siderite make up the carbonate laminae. The Mg content of the primary

carbonates records variations in Mg/Ca ratios in lacustrine waters. d18O and

d13C covariance trends in calcite reinforce closed drainage conditions. d18O

data indicate that the lake system changed rapidly from short-lived

isotopically light periods (i.e. from seasonal to pluriannual) to longer steady-

state periods of heavier d18O (i.e. from pluriannual to millennial). The small

d13C changes in the covariant trends were caused by dilute inflow, changing

the contributions of dissolved organic carbon in the system and/or internal

variations in lacustrine organic productivity and recycling. In both shallow

and deep carbonate facies, sulphate reduction and methanogenesis may

account, respectively, for the larger negative and positive d13C shifts recorded

in the early diagenetic carbonates (calcite, dolomite and siderite). The

lacustrine system was very susceptible to high-frequency, climatically forced

water balance variations. These climatic oscillations interfered with the low-

frequency tectonic and morphological changes in the basin catchment. This

resulted in the superposition of high-order depositional, mineralogical and

geochemical cycles and rhythms on the lower order lacustrine infill sequence.

Keywords Carbonates, closed drainage, hydrology, lacustrine environment,rhythmites, stable isotopes.

INTRODUCTION

Sedimentological, mineralogical and geochemicaldata from ancient and recent lacustrine sequencescan be used in palaeoclimatic and palaeoenvi-ronmental interpretation. Geochemical analysesof carbonate minerals are commonly used for

palaeoenvironmental purposes, given that thegeochemical composition of primary lacustrinecarbonates records the complex interplay bet-ween basin hydrology, morphology and biogenicproductivity (e.g. Talbot, 1990; Talbot & Kelts,1990; Johnson et al., 1991; Valero-Garces et al.,1995; Li & Ku, 1997; Utrilla et al., 1998).

Sedimentology (2002) 49, 1073–1094

� 2002 International Association of Sedimentologists 1073

Climate is one of the major allogenic controlson short-term hydrologic change in lakes and theresulting mineralogical and geochemical records.Moreover, tectonics and drainage evolution alsoexert a major influence on lacustrine basin mor-phology and hydrology and can significantlychange the mineralogical and geochemicalrecord. Nevertheless, the response of lacustrinesystems to simultaneous climatic and tectonic–morphological forcing is poorly understood,especially in large lacustrine systems, wherethese factors interact over large areas and resultin complex depositional records.

The study of ancient, tectonically forced, smalllacustrine systems may help to unravel someaspects of the long- to short-term influence oftectonics, morphology and climate on the evolu-tion of lakes. Small lacustrine systems are verysensitive and readily affected by even minortectonic or climatic changes. Owing to theirreduced dimensions, it may be easier to constrainthe relative influence of interacting tectonics,drainage evolution and climate on their depos-itional records. Following this approach, thispaper deals with the analysis of the depositional,palaeoenvironmental and palaeohydrologicalevolution of the Oligocene As Pontes lacustrinesystem. One of its aims is to establish theresponse of this system to the short- to long-term(i.e. from annual to several hundred kyr) tectonic,morphological and climatic changes that affectedthe As Pontes basin catchment system.

GEOLOGICAL SETTING

The Tertiary As Pontes Basin is a small (12 km2)alluvial–lacustrine basin that developed in aNW- to SE-oriented dextral strike-slip fault systemin north-west Spain (Cabrera et al., 1996). It isbounded to the north by E–W to NW–SE orientedthrusts, whereas its southern margin is defined byan unconformity between the up to 400 m thickTertiary basin infill and the Precambrian–Palaeo-zoic metamorphic basement (Fig. 1). The earlyevolutionary stages of the basin took place in arestraining overstep (Fig. 2), which later evolvedinto a restraining bend situation (Ferrus, 1998).

The palaeobiological record of the As PontesBasin demonstrates that the Oligocene palaeocli-mate in NW Spain was subtropical, warmer thanthe present and probably with dry and rainyseasons (Medus, 1965; Baltuille et al., 1992;Cabrera et al., 1994). The heavy mineral assem-blages yielded by the alluvial facies suggest that

the basin catchment drainage spread significantlyfrom the early to the late basin evolutionarystages, although the lithology of the metamorphicand igneous rocks in the source areas was similarto the present (Barso et al., 2000). Therefore, itcan be assumed that the regional water chemistrywas also similar, although with minor differencesresulting from diverse climatic, drainage andrelief conditions. Present surface run-off andgroundwater inputs are characterized by majorHCO3

– and Ca2+ with lesser amounts of Mg2+, Cl–,SO4

2– and Na+ (Fern�aandez et al., 1983).The basin infill developed over 7 My mainly

from Late Oligocene to Early Miocene (Huertaet al., 1997). It is divided into five major units onthe basis of the relative abundance of lacustrineand marsh deposits and of the geometric tectono-sedimentary relationships between the infillingsequences and the tectonic structures in the basin(Cabrera et al., 1996; Ferrus, 1998). Thus, Units 1,2 and the lower part of Unit 3 filled two maindepocentres (eastern and western), separated by atectonic threshold bounded by an E–W thrust anda N–S normal fault (Fig. 2a and b). The upper partof Unit 3 and Units 4 and 5 gradually spreadthroughout the basin and finally buried thisthreshold and the previously active tectonicstructures (Fig. 1b). Unit 1 records widespreadlacustrine and/or swamp conditions, whereasUnits 2–5 record mainly alluvial and swampzones with no significant lacustrine deposition.

According to magnetostratigraphic data, thesedimentation of the alluvial–lacustrine assem-blages in Unit 1 lasted up to 1Æ5 My (Huerta et al.,1997). This deposition took place in two sub-basins, which were controlled by active thrustsand normal faults and surrounded by smallalluvial fans (Figs 1, 2a and b). This tectono-sedimentary arrangement, together with the over-all water input/output ratio, gave rise in theeastern subbasin to a closed drainage lacustrinesystem (Ferrus, 1998). Unit 1 consists of up to100 m of sediment that accumulated along thenorthern sub-basin areas, whereas the unit wed-ges towards the southern basin margin and onlapsthe basement, which is locally overlain by thinbasal colluvial breccias (Fig. 2c, assemblage Sc).This unit is split into two subunits (1A and 1B,Fig. 2c) and includes five major facies assemblag-es (Table 1; Figs 2 and 3). Assemblage 1 (gravels,sandstones and mudstones) records depositionon small, marginal alluvial fans. Assemblage 2(cyclically arranged mudstones, marls, carbonatesand rhythmites) constitutes most of Subunit 1Aand records deposition in shallow lacustrine–

1074 A. S�aaez and L. Cabrera

� 2002 International Association of Sedimentologists, Sedimentology, 49, 1073–1094

palustrine ramps. Assemblage 3 (laminatedorganic-rich facies) records deeper lacustrinedeposition and constitutes most of lower Subunit1B, which overlies a laterally extensive erosivesurface developed at the top of Subunit 1A. Thecoaly bioclastic mudstone facies (assemblage 4)constitutes the upper part of Subunit 1B andrecords sedimentation on marginal lacustrinebenches that prograded through gentle, low-gradient basinward slopes into the open offshorelacustrine zones (Fig. 3b). Assemblage 4 includesan olive green, fine-grained bed, which is a goodreference level to correlate well cores. Finally, themudstones, coaly mudstones and coal of assem-blage 5 record the development of marshes at thetop of lacustrine benches (Cabrera et al., 1995).

The lacustrine assemblages 3 and 4 are charac-terized by the widespread occurrence of primaryand early diagenetic carbonate minerals, suitablefor analysing the trends of the basin palaeo-hydrology. The following sections are dedicatedto the detailed description and interpretation ofthis record.

SAMPLING AND ANALYTICALTECHNIQUES

The massive palustrine–lacustrine facies in Unit1 were sampled from continuous cores from fivewells (Fig. 2c) with an average sample spacing of20 cm. The laminated lacustrine facies weresampled with similar spacing by extracting clayand carbonates from 17 clay–carbonate laminatedbundles. The carbonate laminae were sampledusing a microdrill.

Fifty selected carbonate and clay samples werestudied using scanning electron microscopy(SEM; Hitachi S2300 and Jeol JSM840, equippedwith an AN10000 Link energy-dispersive andX-ray microanalyser). Energy-dispersive analyseswere carried out to check mineralogy and chem-ical composition.

The quantitative estimation of carbonate con-tents was carried out by decomposition of thesamples with HCl and titration of excess acidwith NaOH using a 702SM Trinitro automatictitrator. Bulk mineralogical composition of 189

Fig. 1. (a) Geological map of the AsPontes strike-slip restraining bendbasin showing the main tectonicstructures that affect the basement.Note the strike-slip fault and asso-ciated thrusts, which bound thenorthern basin margin, the N–S ori-ented normal faults and the E–Wand NE–SW oriented thrusts. Seelocation of wells represented in thecross-section in Fig. 2c. (b) Strati-graphy of the basin showing themain depositional facies and strati-graphic units (see arrows for loca-tion in Fig. 2a).

Closed Oligocene lacustrine system, As Pontes Basin 1075


carbonate samples was determined by X-raydiffraction (XRD) using a Siemens D500 diffrac-tometer. The composition of carbonate sampleswas also determined by standard XRD semi-quantitative methods (Chung, 1974).

The molar percentage of Mg in calcites anddolomites was determined using X-ray diffracto-grams, by measuring the position of the d104 peakrelative to an internal quartz standard (Lumsden,1979). When the sample volume was sufficient

Fig. 2. Palaeogeographic sketches of the As Pontes Basin during Unit 1 deposition. (a and a¢) Lower part of Unit 1(Subunit 1A) characterized by a minor water body and the development of palustrine and shallow lacustrine rampenvironments. (b and b¢) Upper part of Unit 1 (Subunit 1B) characterized by a larger water body and the spreading ofdeeper lacustrine environments fringed by prograding benches. Note the persistence of marsh–swamp environmentsin the western sub-basin. (c) Transverse stratigraphic cross-section of the eastern sub-basin showing the faciesassemblage relationships in Unit 1 (see location triangles in a and b). See well locations in Fig. 1a.



Table

1.

Sig

nifi

can

tse

dim

en

tary

facie

san

dass

em

bla

ges.

Facie

sass

em

bla

ges

Inte

rpre

tati

on

5.

Coal

an

dco

aly

faci

esM

ars

h–sw

am

pzon

es

develo

ped

at

the

top

of

lake

ben

ch

es

Inte

rbed

ded

lign

ite,

coaly

mu

dst

on

ean

dm

ud

ston

ebed

s

4.

Coaly

bio

clast

icm

ud

ston

esL

ake

ben

ch

Dark

,m

ass

ive

dm

-th

ick

bed

s.F

resh

wate

rgast

rop

od

san

dost

racod

sT

OC

36%

.F

ine-g

rain

ed

san

dst

on

ela

min

ae

(TO

C12%

)

3.

Lam

inate

dorg

an

ic-r

ich

faci

esO

pen

-off

shore

(in

ner)

deep

er

lake

Rew

ork

ed

fresh

wate

rfa

un

a:

gast

rop

od

s(L

ym

naea

,P

lan

orb

is),

Low

sali

nit

y.

Mero

mix

is.

Maxim

um

dep

th10–15

mfr

ogs

an

dall

igato

rre

main

s.C

last

icil

lite

,kaoli

nit

e,

qu

art

zan

dfe

ldsp

ar

3c.

Th

ick

cla

y–carb

on

ate

rhyth

mit

es.

1–2

cm

thic

k,

bla

ck

cla

y–w

hit

ecarb

on

ate

cou

ple

tsgro

up

ed

incm

-th

ick

bu

nd

les.

Ch

an

gin

gbio

cla

stic

con

ten

t.P

rim

ary

an

dearl

yd

iagen

eti

cL

MC

,H

MC

an

dearl

yd

iagen

eti

cp

inkis

hsi

deri

tela

min

ae.

TO

C27–29%

.C

alc

ite

geoch

em

istr

y:

Mg

from

3Æ1

to16Æ2

%;

d18O

from

)1

to+

2Æ9&

;d1

3C

very

vari

able

from

)3

to+

11&

.

3b.

Th

incla

y–carb

on

ate

rhyth

mit

es.

0Æ1

–2

mm

thic

k,

bla

ck

cla

y–w

hit

eA

nn

ual–

plu

rian

nu

al

cla

y–carb

on

ate

cou

ple

tsse

dim

en

tati

on

carb

on

ate

cou

ple

ts,

gro

up

ed

incm

-th

ick

bu

nd

les.

Pri

mary

,n

on

-bio

gen

iccli

mati

crh

yth

ms.

Carb

on

ate

pre

cip

itati

on

inep

ilim

nio

nw

ate

rs.

LM

Can

dH

MC

.S

pon

ge

spic

ule

san

dd

iato

mit

ed

isso

luti

on

.E

arl

yd

iagen

eti

cfr

am

boid

al

pyri

te.

TO

C21–31%

.C

alc

ite

geoch

em

istr

y:

Mg

from

0Æ1

to16Æ2

%;

good

d18O

–d1

3C

covari

an

ce;d1

8O

from

)10Æ7

to+

2Æ6&

;d1

3C

from

)4

to+

3&

.

3a.

Fin

e-g

rain

ed

turb

idit

es.

0Æ3

–1

cm

thic

k,

bla

ck

silt

–cla

yla

min

ae

in2

toU

nd

erfl

ow

sfr

om

dis

tal

all

uvia

lfa

nzon

es.

40

cm

thic

kbu

nd

les.

Som

ebed

sw

ith

vegeta

ld

ebri

s,ost

racod

san

dgast

rop

od

bio

cla

sts.

2.

Cycl

icall

yarr

an

ged

mu

dst

on

es,

marl

s,ca

rbon

ate

san

drh

yth

mit

esS

hall

ow

lake–p

alu

stri

ne

ram

pd

m-

tom

-th

ick

cycl

es.

Carb

on

ate

s:L

MC

,H

MC

,d

olo

mit

e.

Cla

stic

0to

afe

wm

of

dep

th.

Holo

mix

is.

illi

te,

kaoli

nit

e,

Al-

smecti

te,

qu

art

zan

dfe

ldsp

ar.

Mg-r

ich

cla

ys:

Oxic

bott

om

con

dit

ion

s.H

igh

sali

nit

yp

aly

gors

kit

e,

sep

ioli

te.

TO

C<

1%

.W

idesp

read

fram

boid

al

pyri

te.

bu

tn

oevap

ori

tic

dep

osi

tion

.

2c.

Dolo

ston

es

an

dli

mest

on

es.

Wh

ite,

ch

alk

y,

mic

riti

c,

mass

ive.

Su

baeri

al–

subaqu

eou

s,p

alu

stri

ne.

Vari

able

cla

ycon

ten

t.M

ost

lyd

iagen

eti

ccarb

on

ate

s:d

olo

mit

eP

ed

ogen

icp

rocess

es.

(dolo

ston

es)

an

dH

MC

,L

MC

(lim

est

on

es)

.R

oot

traces.

2b.

Mu

dst

on

es

an

dm

arl

s.D

ark

top

ale

gre

en

,bio

turb

ate

d,

mass

ive

toS

ubaqu

eou

sla

cu

stri

ne.

poorl

yst

rati

fied

,so

meti

mes

nod

ula

rin

the

tran

siti

on

toF

acie

s2c.

Most

lyd

iagen

eti

ccarb

on

ate

s:H

MC

,L

MC

(dolo

mit

e).

Ath

ala

ssic

fau

na:

Hyd

robia

,P

ota

mid

es.

2b,

2c

carb

on

ate

geoch

em

istr

y:

%M

g:

1Æ6

–16Æ2

%(c

alc

ite),

Calc

ite–ara

gon

ite

pri

mary

mu

ds

42Æ0

–46Æ6

%(d

olo

mit

e),

Sr

avera

ge:

1400

p.p

.m.

(calc

ite),

500

p.p

.m.

(dolo

mit

e).

Narr

ow

d18O

ran

ge

from

)1Æ8

to+

1Æ9&

(LM

C),

from

)0Æ7

to+

3Æ1&

(HM

C),

from

)0Æ9

to+

5Æ4&

(dolo

mit

e);

larg

er

d13C

ran

ge

from

)11Æ6

to)

4Æ2&

(LM

C),

from

)10Æ6

to+

2Æ1&

(HM

C),

from

)6Æ3

to+

4Æ1&

(dolo

mit

e).



and the carbonate content > 10%, the Sr contentof the carbonates was determined for non-lamin-ated, monomineral samples by means of atomicabsorption spectrometry. The carbonate wasdigested in 5 N HCl for 24 h.

The total organic carbon (TOC) content of 108samples was determined with an NA1500 ele-mental organic analyser, coupled to a gas chro-matograph.

Oxygen and carbon isotopic relationships wereestablished on monomineralic inorganic carbon-ate samples or on samples with <5% of othercarbonate minerals. Isotopic analyses were car-ried out in a VG-Isotech SIRA-II� spectrometer.Organic matter was removed by roasting in avacuum oven at 400 �C for 4 h. Sulphur wasextracted through reaction with AgPO4.CO2

(McCrea, 1950) and H3PO4 at 25 �C for 3 h forcalcite samples and at 50 �C for 12 h for dolomitesamples. Analytical precision for the carbonatestandard measurements was ±0Æ02& for d13C and±0Æ12& for d18O. The isotopic data are expressedin delta notation relative to PDB.

LACUSTRINE–PALUSTRINE CYCLICALRAMP DEPOSITS

Facies description

Facies assemblage 2 (Table 1), which includesscarce thin laminated clay–carbonate couplets(Facies 2a), mainly consists of massive greenmudstones and marls (Facies 2b), which alternatewith whitish, chalky limestone and dolostonebeds (Fig. 3a, Facies 2c). The mudstone–carbon-ate cycles range from a few decimetres up to a fewmetres in thickness, with maximum frequencyaround 0Æ5 m, and can be truncated or symmet-rical (Fig. 3a).

The lacustrine green clay–carbonate coupletFacies 2a includes carbonate laminae made up ofpolyhedral and heterometric crystals, rangingfrom a few to 10 lm in size. The couplets aregrouped into thin bundles of laminae, whichoccur at different stratigraphic locations (Fig. 2).The mudstones and marls (Facies 2b) often showmonospecific gastropod accumulation horizons(Hydrobia) in addition to other gastropods(Potamides) and minor remains of charophytes.Mudstones are organic poor with the samplesyielding TOC values below 1%. The whitishcarbonate beds (Facies 2c) are micritic and showgreenish shades. They lack visible primary struc-tures and are homogeneous except for occasional

Table

1.

Con

tin

ued

.

Facie

sass

em

bla

ges

Inte

rpre

tati

on

2a.

Cla

y-c

arb

on

ate

rhyth

mit

es.

0Æ1

–2

mm

thic

kgre

en

cla

y–w

hit

eIn

ner–

deep

er

lacu

stri

ne

or

pro

tecte

dsh

all

ow

stra

tifi

ed

zon

es.

carb

on

ate

cou

ple

tsgro

up

ed

incm

-th

ick

bu

nd

les.

Pri

mary

HM

C.

Su

baeri

al

desi

ccati

on

ep

isod

es

Dis

tort

ed

lam

inati

on

.C

alc

ite

geoch

em

istr

y:

8Æ6

%M

g;d1

8O

from

)1Æ2

to+

2Æ2&

;d1

3C

from

)4Æ3

to)

5Æ6&

1.

Gra

vel

s,sa

nd

ston

esan

dm

ud

ston

esA

llu

via

l-d

om

inate

dd

ep

osi

ts

1b.

Mu

dst

on

es

an

dsa

nd

ston

es.

Pale

gre

en

mass

ive

mu

dst

on

es

wit

hM

ed

ial

tod

ista

lall

uvia

lfa

nzon

es

aff

ecte

dby

ped

ogen

esi

sou

tsiz

efl

oati

ng

qu

art

zgra

ins

an

din

terb

ed

din

gfi

ne-g

rain

ed

san

dst

on

ebed

s.C

last

icil

lite

,kaoli

nit

e,

Al-

smecti

tean

dfe

ldsp

ar.

Inte

rbed

ded

carb

on

ate

nod

ula

rbed

sw

ith

root

traces.

1a.

Fin

e-g

rain

ed

gra

vels

,coars

esa

nd

ston

es.

Mass

ive.

Pro

xim

al

all

uvia

lfa

nzon

es

Cla

stic

qu

art

zan

dfe

ldsp

ar.

Sc.

Basa

lcoll

uvia

lbre

ccia

sS

cre

ed

ep

osi

tsF

ine-g

rain

ed

bre

ccia

s.T

hin

,le

nti

cu

lar

an

dd

iscon

tin

uou

sbed

sof

mon

ogen

icbre

ccia

s.U

pto

10

cm

ind

iam

ete

rsl

ate

cla

sts.

Cla

stic

illi

te,

kaoli

nit

e,

Al-

smecti

te.



fractures (i.e. vertical and horizontal skew planes,Brewer, 1964; and root traces, Freytet & Plaziat,1982).

Carbonate mineralogy, fabricand Mg–Sr content

The mineralogy of the carbonate laminae inFacies 2a was determined for three laminae froma bundle in the upper part of Subunit 1A (Fig. 5,bundle 1). High Mg Calcite (HMC) was the onlycarbonate phase recorded, with an average Mgcontent of 8Æ6%.

Most of the mudstone–carbonate cycles (Figs 3aand 4a) show a clear predominance of Low MgCalcite (LMC) or HMC over dolomite in the lowermudstone beds in the cycles, whereas morevaried occurrences of LMC, HMC and dolomitecharacterize the carbonate beds, with dolomitedominant in dolostones (Figs 3a and 5). Sideriteis scattered and occurs in small amounts in 25%of the samples. A small amount of bioclasticaragonite is also present (accumulations of gas-tropod shells in the mudstones).

SEM images of the marl beds (Facies 2b) showscattered calcite and dolomite crystals embeddedin a clay-dominated matrix. Some of the calcitecrystals are euhedral with equigranular rhombo-hedral habits, ranging from 1 to 5 lm in size,

whereas others do not have well-developed faces.Dolomite crystals are readily recognized despitetheir small size (from 0Æ8 to 2Æ5 lm); they aretypically single homometric and with a rhombo-hedral or polyhedral habit. Palygorskite is wide-spread and displays a clustered-fibrous habit,with fibres clearly adapting to the dolomitecrystal surfaces (Fig. 4c).

The dolostones and limestones (Facies 2c) aremassive, fine-grained (�3 lm crystals) beds.Limestones generally have anhedral–subhedralcalcite. Dolostones display homometric subhe-dral–euhedral dolomite crystals with a goodintercrystalline porosity (Fig. 4b).

The Mg content in calcites from Facies 2b and2c ranges from 1Æ6% to 16Æ2%. Dolomites are non-stoichiometric and Ca rich with MgCO3 rangingfrom 42Æ0 to 46Æ6 mol% (Fig. 5). The Sr content ofthe calcites ranges from 400 to 1500 p.p.m.,whereas dolomites have lower values(500 p.p.m. on average). The calcite Mg and Srcontent trends are concealed and altered in themiddle part of Subunit 1A by the superimposeddolomitization (which interrupts the calciterecord) and the presence of four bioclastic sam-ples, which include aragonite (resulting in anom-alous Sr maxima ranging from 2500 to4000 p.p.m.; Fig. 5). Nevertheless, a Mg–Srenrichment trend occurs in the lower part of the

Fig. 3. (a) Palustrine–shallow lacustrine-dominated ramp model for deposition of the cyclically arranged mudstonesand carbonates. (b) Depositional model of the deeper, lacustrine-dominated evolutionary stage. Both models displaythe ideal sequential arrangement of the most widespread facies, which are displayed here with their main minera-logical characteristics.



subunit, whereas a Mg–Sr depletion trend isrecorded in the upper part (Fig. 5).

Oxygen and carbon isotopic results

The d18O values recorded in the carbonate lam-inae from Facies 2a (bundle 1) range from )1Æ2 to+2Æ2&, whereas the d13C values range from )4Æ3 to)5Æ6& (Table 1, Fig. 6). The d18O values in thecalcites from Facies 2b and 2c range from )1Æ8 to+3Æ1& (Figs 5 and 6). Dolomites have higher d18Ovalues (from )0Æ9 to +5Æ4&). The d13C values ofcarbonate minerals in the mudstone–carbonatecycles show a wider range of variation (from)11Æ6 to +4Æ1&). LMC, HMC and dolomites haveaverage d13C values of )7Æ5, )5Æ0 and )0Æ9&respectively (Fig. 6).

Mineral composition of the (originaland early diagenetic) carbonate muds

The textural, mineralogical and geochemical fea-tures of the carbonate crystals and crystal clustersthat make up the carbonate laminae of Facies 2a,compared with the carbonate laminae describedpreviously in other case studies (Hsu & Kelts,1978; Anadon et al., 1988), show that they aremainly primary in origin and resulted from HMCprecipitation in the water column. In contrast, thetextural and geochemical features of the dolomite-and calcite-bearing facies in the mudstone–carbonate cycles (Facies 2b and 2c) and theirclose relation to the palustrine facies suggestan early diagenetic origin linked to interstitial

carbonate precipitation, and to pedogenetic pro-cesses affecting calcite–aragonite-dominated ori-ginal carbonate muds. The probable occurrenceof a significant aragonite-dominated precursor(ADP) in the original carbonate muds is supportedby the high Sr contents (higher than 1500 p.p.m.)in most of the non-bioclastic calcite-dominateddeposits (Lasemi & Sandberg, 1993; Wright et al.,1997).

A number of authors have argued that dolomitecan be formed by direct precipitation at lowtemperatures and under special conditions ofhighly supersaturated waters with a high Mg/Caratio and an elevated carbonate/bicarbonate con-centration (Hardie et al., 1978). Last (1990) inter-preted the origin of most lacustrine dolomite asprimary (deposited directly from the lake watersor from interstitial porewaters), while maintain-ing that very few of the occurrences containadequate petrographical evidence of a true sec-ondary (i.e. replacement) origin. In fact, it isalmost impossible to distinguish between prim-ary and very early diagenetic dolomite in lacus-trine evaporite settings (De Deckker & Last, 1988).The As Pontes dolomites are fine grained,subhedral to anhedral and non-stoichiometric.They show low Dd18ODOL–CAL average values,which are consistent with the experimental dif-ferences in the oxygen isotopic fractionationcoefficient existing between calcite and dolomitewhen secondary dolomite is produced by thereplacement of pre-existing calcite under isotopicequilibrium and low temperature conditions(Fritz & Smith, 1970; Matthews & Katz, 1977;

Fig. 4. (a) Well core showing the palustrine–lacustrine shallowing-deepening trends of Facies Assemblage 2 (Subunit1A). The lighter, whitish palustrine carbonate facies (mostly dolomite) display cracks and root traces. The darker,green beds correspond to shallow lacustrine mudstones and marls. Scale bar on the left is 55 cm in length. (b) SEMview of equigranular, rhombohedral and polyhedral dolomite crystals in the dolostone beds in Facies 2c. (c) SEM viewof scattered dolomite crystals surrounded by palygorskite fibres in carbonate-rich mudstones in Facies 2b.



McKenzie, 1985; Arenas et al., 1997). Accord-ingly, mesoscopic pedogenic features observed inAs Pontes dolostones suggest that dolomitizationtook place in porewaters resulting from hydro-logic pumping and early pedogenesis. Dolomiti-zation would have been favoured by the high Mg/Ca ratio, high alkalinity and low sulphate contentin the porewaters (Cerling, 1996). Low Sr contentin dolomite would have resulted from the ‘puri-

fication’ (i.e. expulsion from lattice) of this cationduring dolomitization (Land, 1985).

Palaeoenvironmental interpretation

The absence of widespread, well-developed lam-ination in these facies suggests complete biotur-bation and deposition under oxic conditions.Some carbonate beds exhibit bioturbation and

Fig. 5. Cyclically arranged palustrine–lacustrine mudstones and carbonates of Subunit 1A in well 642 (see locationin Figs 1 and 2). Note the distinct development of limestone and dolostone beds and the Mg, Sr, d18O and d13Ctrends. The truncated cycles record an upward-increasing carbonate content, which ends with a sharp transition intolacustrine clay–carbonate rhythmites and/or massive green mudstones belonging to the overlying cycle. These car-bonate beds record intense pedogenesis, including vertical, clay-infilled root traces. Symmetrical cycles show moregradual transitions from the carbonate into the mudstone-dominated beds.



traces of subaerial exposure, including root tra-ces. These are typical pedogenic features and areconsistent with a palustrine or marginal shallowlacustrine origin (Freytet & Plaziat, 1982; Platt &Wright, 1991).

The overall depositional, palaeobiological andearly diagenetic features in this cyclical assem-blage record repeated deepening–shallowingevents linked to lacustrine expansion and con-traction, which affected low-gradient, mostlymarginal lacustrine ramps (Fig. 3a). The green,massive mudstones and marls correspond tobioturbated subaqueous lacustrine deposits, dom-inated by fine-grained contributions from thedistal alluvial mudflats that fringed the lacustrineareas. These deposits record early water inflowand sediment input generating a shallow andpersistent, holomictic water table. This watertable could be fresh to dilute, but the frequentoccurrence of monospecific Hydrobia accumula-tions and the saline water-adapted gastropodPotamides (Hyman, 1967; Anadon, 1989, 1992;

Plaziat, 1993) suggests that lacustrine watersquickly became saline (but not hypersaline).Moreover, the low organic matter preservationand the d13C-depleted values of the carbonateminerals suggest that these deposits formedunder oxic bottom conditions and were affectedby a subsequent anaerobic degradation of organicmatter (via sulphate reduction) during shallowburial diagenesis.

The sporadic laminated beds of Facies 2apresent in the cyclical assemblage of Subunit1A record the development of open offshore,relatively deep and/or sheltered lacustrine zoneslinked to rising and highstand water levels ordeposition in protected shallow saline, stratifiedzones. The first interpretation is more probablegiven that the thicker laminated beds were oftendeposited in the deeper lacustrine zones, whichdeveloped close to the more rapidly subsiding,northern basin zones (Fig. 2a and b). Preserva-tion of these deposits in shallower lacustrinezones was rather low, as evidenced by theircontortion and disruption, caused by desiccationand bioturbation.

The wide extent of the palustrine carbonatesand the restricted occurrence of finely stratified orlaminated clay–carbonate facies attributed todeeper lacustrine deposits suggest that the bottomgradient was low and that the palustrine–lacus-trine carbonate facies spread over the entire basinduring water-level lowering. Nevertheless, atleast during some high water level depositionalepisodes, the palustrine carbonates were depos-ited on marginal low-gradient ramps differenti-ated from the offshore lacustrine zones (Fig. 3a).

OPEN OFFSHORE, DEEPERLACUSTRINE FACIES

Description

Facies 3a

Fine-grained turbidites constitute centimetre- todecimetre-thick beds, most of which are made upof thinning-upwards, 2 to 40 cm thick bundles(Fig. 7a, Table 1). Some of the coarser bedsinclude sand-sized vegetal debris (phytoclasts)and ostracod and gastropod bioclasts. Thismaterial was derived from underflows, whichspread from distal alluvial fan zones and marginalvegetated lacustrine benches and reached theoffshore zones (Figs 2a, b and 3b). Facies 3aoccurs more frequently in the lower part ofSubunit 1B, where it is interbedded with Facies

Fig. 6. d18O and d13C values of primary and early dia-genetic carbonates in Subunit 1A (wells 642 and 5003).See text for further explanation and location of bundle1 in Figs 5, 9 and 10.



3b and 3c deposits, and is better developed in thenorthern basin sequences.

Facies 3b

Thick, laterally continuous clay–carbonate, mic-rolaminated varve-like couplets (0Æ03–1Æ4 mm inthickness) are especially common in the lowerpart of Subunit 1B (Table 1). The thickest laminaebundles are oil shales (TOC 21–31%) and can be

traced basinwide. The carbonate laminae aregrouped into centimetre-thick bundles (Fig. 7a–c)made up of 0Æ1 to 2 mm thick couplets andseparated by thin (3a) turbidite beds. The claylaminae are dark and made up of detrital clayminerals plus sponge spicules, diatoms, ostrac-ods and small gastropod remains; magnesium-rich clay minerals are absent. The carbonatelaminae are whitish and chalky and show thepredominance of inorganic, non-biogenic crystalsand minor fauna and flora remains. Widespreadcorrosion of diatom frustules and sponge spiculesand the replacement of diatom silica by LMC(Fig. 9) suggest relatively high pH conditions(Brookins, 1988). Moreover, early diageneticpyrite and the circular dissolution structuresrecognized in the sponge spicules (Fig. 7b) recordbioerosion, probably related to bacterial, fungal ordiatom activity (as described by Andrejko et al.,1983 and Wuttke, 1992).

Facies 3c

This often occurs in the middle and upper part ofSubunit 1B (Fig. 4). This facies differs from 3b inhaving thicker, less continuous carbonate lam-inae and widespread bioclastic laminae. Somepink laminae include significant amounts of earlydiagenetic siderite (Table 1).

Carbonate fabric, mineralogy and Mg content

As in similar cases studies (Hsu & Kelts, 1978;Anadon et al., 1988) calcite (HMC, LMC) is themost widespread primary and early diageneticcarbonate mineral in the rhythmite facies. Crys-tals are polyhedral and heterometric, rangingfrom 7 to 10 lm in size (Fig. 8).

Most of the carbonate laminae in Facies 3b and 3cconsist of HMC, although two of the laminae bun-dles include LMC laminae (Fig. 9). HMC crystals

Fig. 7. (a) Open offshore, laminated lacustrine faciesof Subunit 1B. Dark, fine-grained turbiditic beds (Facies3a) alternate with lighter bundles of carbonate–clayrhythmites (Facies 3b). (b) Sponge spicule showingcircular dissolution pits attributed to bacterial, fungalor diatom boring activity (bundle 15, Facies 3c). Seetext for explanation and references. (c) Close-up ofclay–carbonate rhythmite bundles (bundles 11 and 12,Facies 3b). Note the changing laminae thickness andthe syndepositional microfaults that affect the bundles.See text for further explanation.

Fig. 8. Carbonate textures of lamin-ated lacustrine facies of Subunit 1B.(a) SEM view of arrow-like HMCcrystals (bundle 7, Facies 3b).(b) SEM view of clustered HMCcrystals (bundle 3, Facies 3b).(c) SEM view of rhombohedral HMCcrystals showing dissolutiontextures (bundle 14, Facies 3 c).



show rhombohedral shapes (Fig. 8c), and somedisplay an arrow-like morphology (Fig. 8a and b).In some cases, crystal faces are either corroded orshow additional growth (Fig. 8c). Siderite crystalsin these facies are rhombohedral, 1–2 lm in size,with well-preserved crystal faces. They occur asclusters with a similar crystal orientation and infillintercrystalline calcite porosity.

The Mg content in the LMC ranges from 0Æ1 to3Æ8 mol%, whereas in the HMC, it ranges between4Æ6 and 16Æ2 mol%. The average Mg content of thecalcites in each clay–carbonate laminae bundle inwell 642 increases upwards in the lower part of

Subunit 1B (Fig. 9). Moreover, most of the bundlesshow clear increasing %Mg trends, which arearranged in pluriannual cycles. These trends inMg increase do not correlate significantly with thelamina thickness or with the oxygen and carbonisotopic changes described below (Fig. 10).

Oxygen and carbon isotopic results

The d18O values of most of the carbonate laminaein Facies 3b range from )1 to +2Æ6&, but a fewlaminae have lower values (Fig. 11b). The d13Cvalues are between )4 and +3&. Covariance

Fig. 9. Mg, d18O and d13C profiles of the HMC, LMC and siderite from the carbonate–clay rhythmites in Subunit 1B(correlated wells 642 and 728). The average mol% Mg in the bundles displays a trend that suggests increasingsalinity. The d18O values show a clear shift to positive values, punctuated by dilution episodes with lighter d18O(bundles 2–10). The d18O and d13C values show a clear covariant trend in the lower part of the subunit (bundles 2–10)pointing to closed hydrological conditions. Bundle 1, Facies 2a. Bundles 2–14b, Facies 3b. Bundles 14–17 (blacksquares), Facies 3c.



between d13C and d18O is high. Calcite in thethicker Facies 3c laminae show slightly changingd18O values (average +0Æ7&; Figs 9 and 11a). Thed13C values are clearly more variable than inFacies 3b. Some of the bundles display low d13Cvalues (between )3 and +11&). No clear sequen-tial trends can be recognized in these changes.Finally, d18O and d13C do not show covariance,and d18O and the % Mg do not correlate.

Palaeoenvironmental interpretation

The open, offshore lacustrine assemblage facieswas deposited initially at a maximum depthranging from 10 to 15 m, although this depthdecreased as the basin infill progressed (Fig. 3b).This assemblage is regarded as deep, as opposedto the aforementioned shallow-dominated faciesassemblage. Maximum depth estimation was

established on the basis of: (1) the thickness ofthe laminated lacustrine basin infill episode; and(2) the height of gentle progradational marginalclinoforms observed in the stratigraphic panels(Fig. 2c; Cabrera et al., 1995; Ferrus, 1998).

The widespread occurrence of laminated faciesand the good organic matter preservation showthat the water body was perennial and sufficientlydeep to allow long periods of water stratification(meromixis). The carbonate–clay rhythmite inter-vals record repeated limnological changes, whichled to alternating deposition from: (1) carbonateprecipitation from warm surface waters; and (2)suspensions of clay biogenic clasts and organicmatter. The microlaminated clay–carbonate cou-plets of Facies 3b are consistent with brief, high-frequency climatic rhythms, probably seasonal,although conclusive evidence of such periodicityis not available. The grouping of the clay–carbonate

Fig. 10. (a) Succession (not to scale) of the clay–carbonate rhythmite bundles 1–12 from the lower part of Subunit1B, showing Mg, d18O and d13C of the primary carbonates (see Fig. 9 for stratigraphic location of the bundles). (b) Thesame trends are shown in detailed sedimentological logs of some selected rhythmite bundles (9, 10 and 12).The trends of increasing Mg content record pluriannual to several thousand-year-long cycles of increasing soluteconcentration. The lighter d18O values point to significant dilution episodes caused by major water inflows.



couplets in bundles and their alternation withturbidites suggest successive pluriannual to cen-tennial periods of changing hydrochemistry andfine-grained sediment inputs. These changescould be related to a variety of factors, eitherregional (i.e. changing rainfall and vegetationcover) or local (i.e. shifting of terrigenous contri-butions from the alluvial zones).

EVOLUTION OF THE LACUSTRINESYSTEM

Basin fill evolution and lacustrine–palustrinecarbonate mineralogy

The eastern As Pontes sub-basin fill sequencerecords an early period of basin formation withrelated scree and alluvial deposits. This was

followed by the development of palustrine toshallow lacustrine terrigenous sedimentation(Table 1, Fig. 5) These palustrine-dominated en-vironments evolved into a more diverse frame-work, in which terrigenous carbonate cyclicalfacies were deposited. Areally extensive andrelatively deep, open offshore lacustrine zonesdeveloped during this stage, but they oftenshrank, and shallow lacustrine and palustrineramps spread over most of the basin. This resul-ted in low generation and poor preservation oforganic-rich and laminated facies.

The sedimentological, mineralogical and palae-obiological records in the lower subunit demon-strate that the water depth and salinity werehighly variable, characterizing closed lacustrineconditions. The possible occurrence of primaryaragonite and the widespread early diageneticdolomite indicate that the Mg/Ca ratio attainedrelatively high values. This high Mg/Ca ratioresulted from the Mg contribution from theweathering of metamorphic rocks containing Mgsilicates. The removal of Ca during the earlystages of lacustrine evolution (corresponding tothe deposition of LMC and HMC in the lowerlacustrine cycles) contributed to the relativelyhigh Mg/Ca ratios, which remained high despitethe generation of Mg-rich clays (i.e. palygorskiteand sepiolite). Moreover, the fact that the mostsignificant dolomite accumulations are linked topedogenic palustrine facies suggests that the Mg/Ca ratio increased as a result of evaporitic pump-ing. Neither dolomite (early diagenetic or prim-ary) nor aragonite is indicative of a saline lakeformed under evaporative conditions (Muller &Wagner, 1978). Nevertheless, in As Pontes, higholigosaline to mesosaline and even eusalineconditions are confirmed by the athalassic faunalrecord.

The palustrine–lacustrine, ramp-dominateddepositional framework of Subunit 1A wasreplaced by a deeper, meromictic and morecomplex lacustrine framework, ranging fromrelatively uniform deeper lacustrine environ-ments (with fine-grained turbidites and clay–carbonate couplets) to marginal bench–talusenvironments (Subunit 1B). Primary HMC wasthe dominant carbonate phase in the openoffshore lacustrine deposits during this evolu-tionary stage, whereas LMC and siderite were themost common early diagenetic carbonates.

The occurrence of primary LMC in the lower-most clay–carbonate bundles suggests that theMg/Ca ratio fell during major flooding events.This ratio increased during the successive depo-

Fig. 11. d13C and d18O of the carbonate laminae fromSubunit 1B (wells 5040 and 728). (a) Arrow indicatespositive shifts in the d13C values recorded in sideriteand some HMC laminae from the upper part of thesubunit, which suggests a methanogenic influence.(b) The covariant trends (labelled L1, L2 and L3)recorded in the lower part of the subunit point toclosed hydrological conditions.



sitional stages, to remain relatively high. Thechanges in primary carbonate mineralogy (i.e.from LMC to HMC) reflect the early precipitationof Ca-carbonates (mostly LMC), which caused Caremoval from the lake waters and led to periods ofrelatively higher Mg/Ca waters, owing to the moreconservative character of Mg and the hydrologi-cally closed conditions (Muller & Wagner, 1978;Wetzel, 1983). The absence of primary aragoniteand the occurrence of fossil frogs and typicalfresh to slightly oligosaline water snails (Lym-naea, Planorbis) suggest that the Mg/Ca ratio andsalinity never attained very high values.

Swamp environments also developed in themarginal lacustrine zones and prograded basin-ward, resulting in the generation of several pro-gradational–retrogradational sequences, whichpunctuated the basin infill evolution. The upperpart of the basin infill sequence records the latebasinward progradational infilling of the offshorelacustrine zones by encroachment of the marginalswamps. This stage was characterized by increas-ing higher plant contributions to the subaqueouslacustrine zones, which underwent a gradualdecrease in areal extent and water volume.

Carbonate isotopic evolution

Cyclically arranged palustrine–lacustrineassemblage

Primary HMC was recognized in the scarce,deeper lacustrine clay–carbonate couplets recor-ded in the lower Subunit 1A, the only primarycarbonate phase recorded there. Some of the LMCand HMC that cluster close to the isotopic valuesof the primary HMC might also be primary, but noevidence is available. Thus, it is assumed thatfine-grained LMC, HMC and dolomite are theearliest diagenetic carbonate minerals in themassive lacustrine–palustrine sequences in Sub-unit 1A (Fig. 5).

Dolomite is enriched in 18O in relation tocalcite by between +1 and +7& (Clayton et al.,1968; Fontes et al., 1970; Land, 1985). Thesedolomite 18O values are consistent with precipi-tation from similarly evolved lacustrine waters(Craig & Gordon, 1965). Approximately one-quarter of the LMC samples cluster with negatived18O values (i.e. from )2 to 0), suggesting thateither the evolution of the lacustrine porewaterswas not so pronounced or the input of 16O-richmeteoric waters was higher, compared with thehigher HMC and dolomite d18O values. Some ofthe lowest LMC d18O values occur in the lower-

most cyclical sequences in Subunit 1A, suggest-ing less concentrated water conditions in thebasin (Fig. 5).

The very wide range (i.e. 16&) of d13C valuescould represent early diagenetic precipitation ofcarbonates under varying mixtures of atmo-spheric and soil-derived CO2 from the surround-ing emerged zones. With regard to d13C variations,a number of authors have demonstrated thatgroundwaters in well-vegetated catchment areaswith well-developed soils are more influenced bythe decomposition of isotopically light plantmaterial and have CO2 with negative d13C values(Talbot, 1990). Moreover, changes in the vegetat-ive cover in the source and depositional palus-trine areas could subordinately account for thesevariations. Thus, a decrease in C4 plant contri-butions (d13C, )12 to )14&) and a correlativeincrease in C3 plant contributions (d13C, )25 to)28&) would cause a d13C decrease and viceversa. Some of the changes in the d13C DIC(dissolved in organic carbon) in the residentwaters could also have been controlled by vari-able recycling of organic matter, which wasmodified by burial rate. On the other hand, thelower d13C values of LMC and HMC, ranging from)7 to )12&, would mainly be related to earlydiagenetic precipitation of calcite in associationwith bacterial sulphate reduction, because suchlow values do not usually result from atmo-spheric or soil-derived CO2 contributions (Talbot& Kelts, 1990).

Open offshore deeper lacustrine assemblage

In the As Pontes deeper lacustrine sequences,the d18O values from primary HMC are associ-ated with the hydrological balance in the lake,which depends on the evaporation/inputs ratio.In general, an increase in d18O values in primarycarbonates reflects increasing evaporation orlonger water residence times in the lake. There-fore, the primary calcites with positive orstrongly positive d18O values could representsedimentation during relatively concentrated,longer residence time stages. The d18O and d13Cvalues of the primary carbonates in the lowerbundles (2–12 in Figs 9 and 10) show covarianttrends (L2 and L3 in Fig. 11b) and suggest thatthe lacustrine system was hydrologically closed.The covariant trends have a relatively gentleintermediate slope, suggesting that the area/depth ratio of the basin during the sedimentationof these deposits was high. This accords wellwith the shallowness of the basin (i.e. from a



maximum of 10–15 m to a few metres), estab-lished on the basis of depositional and geometricconstraints.

The low to very low d18O values (between )3Æ2and )10Æ7&), recorded in some of the LMCincluded in the covariant trend of Subunit 1Bsuggest that calcite precipitation occurred duringshort dilution periods owing to a greater influx ofisotopically lighter waters. These low d18O valuesshould be regarded as being representative of themean oxygen isotopic composition of the inflow,which reflects the composition of the catchmentprecipitation.

The inflow responsible for reducing d13C andd18O (up to 7 and 13& respectively) would berelated to large surface run-off and/or under-ground recharge events linked to exceptionalrainfall in the region. The differences in theamount of isotopic lowering between d13C andd18O would result from the fact that d18O in thelacustrine waters decreases following a lineartrend, whereas d13C is especially influenced bythe CO2 isotopic composition of the lacustrineresident waters (Li & Ku, 1997). The significanceof these major flood events would be enhanced bythe relatively small lacustrine water reservoirvolume and by water stratification, which wouldhave resulted in the generation of widespreadmeteoric water plumes in the epilimnion duringthe flood events (Johnson et al., 1991).

Isotopically light laminae in the lower part ofSubunit 1B are scarce compared with the isotop-ically heavy laminae. Most of the carbonatelaminae in the L2 and L3 covariant trends showhigh d18O and d13C values with little variation(Fig. 11b). Thus, the lacustrine system was char-acterized during most of its evolution byan isotopically heavy, gently varying water body,close to steady state (Sofer & Gat, 1975; Listeret al., 1991) and only punctuated by sharp excur-sions to light isotopic values. Shorter term (an-nual to pluriannual) d18O changes (about 2&)recorded in most of the clay–carbonate laminaebundles could be attributed to a variety of mech-anisms (mixing of minor, varying meteoric waterinputs, changes in evaporation rates, water tem-perature changes, organic productivity changes).

The differences between the d13C values in thecovariant trends L2 and L3 (about 3&) could becaused by (Bein, 1986; Talbot, 1990; Valero-Garceset al., 1997; Wachniew & Rozanski, 1997): (1) theinflow and mixing of variable amounts of meteoricwater with isotopically light 12C contributions;(2) changes in d13C in the incoming waters due tovegetation changes in the source areas around the

lacustrine zones; (3) the different temperaturesin the epilimnion (probably less relevant); and(4) variations in biological productivity during abasically stratified lake period on the assumptionthat most of the carbonates precipitated in theepilimnion. It is not possible to assess the import-ance of each of these processes, which probablyacted simultaneously. It has been suggested thatchanges in primary productivity have little impacton the composition of the lake DIC of long-residence water bodies (Turner et al., 1983; Talbot& Kelts, 1990; Utrilla et al., 1998), although this isdebatable. An alternative interpretation for theheavier d13C values is changing rates of net CO2

outgassing from the lake to the atmosphere (acommon process in most recent lakes; Cole et al.,1994). This outgassing would account for theheavier d13C values, which could be attainedunder closed, long-residence water conditions.Varying organic matter production and recyclingthrough oxidation would also account for theobserved d13C variations.

The d18O values of the diagenetic LMC, whichreplaced diatom frustules (bundle 13, Figs 9 and11b), cluster between those of the primary LMCand HMC that precipitated from evolved lacus-trine waters. This suggests that replacement ofsilica took place in these evolved waters ratherthan in meteoric waters. The same explanationcan be stated for siderite d18O values, althoughthey show a slightly larger range. In contrast, thed13C values for early diagenetic LMC and sideriteare rather diverse, with siderite reaching signifi-cantly higher values (bundle 17, Figs 9 and 11a).In many recent lacustrine basins, 13C enrichmentin early diagenetic carbonates is related to bac-terial methanogenesis (Talbot, 1990). The sideriteprecipitation in the uppermost laminite bundlesin upper Subunit 1B was probably related to thisprocess (Berner, 1981; Mozley & Wersin, 1992),which in turn was enhanced by increasingcontributions of vegetal debris and eutrophiza-tion of the lake. Moreover, given the shift towardshigher d13C values of some of the HMC in theupper bundles, it cannot be ruled out that partialearly diagenetic HMC precipitation could takeplace under the influence of methanogenic CO2.

Integration of the isotope data from lowerand upper subunits

The primary HMC from the lower subunit lami-nites has d18O values in the same range as thehigher values in the upper subunit (Fig. 12). Lowd18O values related to dilution episodes are not



recorded in this subunit, but this could be a biascaused by the small number of clay–carbonatecouplets available for sampling and by the rapidshift to the higher isotope values. Consequently,only the d18O heavier stages are identified in thisisotopic record. During the sedimentation ofboth lower and upper subunits, meteoric watersapparently evolved very rapidly from lowerto higher isotopic values.

Most of the dolomite d18O and d13C valuescluster above the range of most of the earlydiagenetic HMC and LMC (Fig. 12). Nevertheless,these values also scatter close to the end of thecovariant trends in the upper subunit and alsoclose to the cluster of primary HMC in the lowersubunit. This suggests that the waters from whichdolomite precipitated in lower Subunit 1A couldhave had d18O values similar to those of theevolved waters recorded in upper subunit 1B.This means that the residence times and/orevaporation rates remained similar throughoutthe evolution of the lacustrine system.

Changes in basin hydrology

Lacustrine establishment and earlysalinity increase

The lower Subunit 1A represents the onset of awell-developed palustrine- to shallow lacustrine-dominated environment (Fig. 2), in which fine-

grained mudstones and palustrine (LMC-domin-ated) limestones were deposited. The Mg and Srcontent changes point to variable solute compo-sition, whereas the relatively high and roughlyupward increase in d18O values suggests highevaporation and/or residence time.

Changing solute trends

A subsequent rise in the Mg/Ca ratio, probablyrelated to a salinity increase, resulted in wide-spread primary and early diagenetic HMC preci-pitation and in dolomite and Mg-rich claygeneration. The lacustrine waters probablyattained mesosaline to eusaline conditions,favouring the development of a thalassoid fauna.The mudstone–carbonate cycles record higherfrequency water level oscillations, probably linkedto salinity changes. Salinity maxima in theremaining water bodies could be coeval with themaximum spreading of the palustrine environ-ments in which dolomitization took place (Facies2c). Salinity minima could be coeval with a rise inthe water level and with the deposition of openoffshore lacustrine carbonate–clay couplets(Facies 2a) and carbonate-rich mudstones (Facies 2b).

Salinity decrease trend and system dilution

A deepening and freshening of the system duringthe deposition of the upper part of Subunit 1A is

Fig. 12. Integrative sketch of d18Oand d13C values of all primary andearly diagenetic carbonate samplesfrom lacustrine Subunits 1A and 1Bin the eastern As Pontes subbasin.The reference sedimentological logon the left is not to scale. The mainprocesses affecting the primary anddiagenetic trends are indicated.Note the location of the carbonatesamples (D, L, M) represented by theclusters in the synthetic log. D,dolomite; L, laminites; M, massivecarbonate mudstones. Arrowsbetween cluster L1 and covarianttrends L2 and L3 suggest the posit-ive d13C trend in successive evolu-tionary stages. See discussion in thetext.



suggested by the occurrence of laminated clay–carbonate couplets and by the decreasing Mgcontent and d18O values of the limestones. Thesechanges heralded a dilution and deepening event,which culminated in the deposition of the lower-most portion of Subunit 1B.

Meromictic lake: freshwater–oligosalineconditions and final basin infill

The water balance of the lake changed in thetransition from Subunit 1A to the overlyingSubunit 1B. Increasing water inflow resulted infalling salinity, as evidenced by the absence ofthalassoid gastropods, which were replaced byfreshwater species, together with the occurrenceof fossil frogs which were poorly adapted tohighly saline conditions. Moreover, HMC (prima-ry and perhaps in part early diagenetic), LMC(primary and early diagenetic) and siderite (earlydiagenetic) are the main carbonates present. Thismineralogy reflects variations in the Mg/Ca ratioand oscillations of the solute composition, whichwere not as important as in the underlying unit.

The lower part of Subunit 1B records a moreexpanded lacustrine framework, with an earlymajor flooding that resulted in a significant watervolume increase and basin deepening and fresh-ening (Fig. 2). The meromictic or oligomicticconditions invoked to account for the laminatedlacustrine facies could be enhanced by the trop-ical–subtropical climate proposed for NW Spainduring the Oligocene on the basis of the availablepalaeobotanical data. Stratification of the waterscould have been reinforced initially by thesuperposition of the incoming meteoric floodwaters on the pre-existing, more saline waterbody during the preceding lacustrine stage (ecto-genic meromixis).

The Mg increase in the lower part of Subunit 1Bsuggests an early water concentration up tooligosaline conditions. The increasing Mg contentin successive carbonate laminae bundles suggestsprogressive increase in the evaporation/waterinput (E/I) ratio, which reflects distinct seasonalto pluriannual cycles (Fig. 10). Moreover, thewide scatter in d18O values and the prevailingclosed basin conditions indicate that the AsPontes lake was especially sensitive to changesin the evaporation/inflow ratio.

The poorer preservation of the geochemical andmineralogical record in the upper part of Subunit1B prevents us from tracing its detailed evolution,although the scarce data suggest that furtheroscillations and solute composition changes took

place before the final sedimentary infilling byprogradation of the marginal parts of the system.

In intermediate to long time ranges, the Mgconcentration trend in Subunit 1B records severalstages of increase and decrease, from the earlylacustrine flooding to a first maximum that wasfollowed by a pulsating evolution (see lowerbundle succession in Subunit 1B; Figs 9 and10). Note that the average salinity trends markedby the Mg increase are not always coeval with theincreasing residence time or E/I ratio, evidencedby the d18O increase. At shorter time intervals(pluriannual to centennial), positive or negativeshifts in d18O (i.e. increasing or decreasing E/I) donot usually parallel changes in the Mg content.Because the Mg concentration tends to be veryconservative on a short time scale (Wetzel, 1983),this would account for the frequent absence ofshort-term correlation between Mg and d18Otrends (Fig. 10).

The d18O and Mg content changes recognized inthe primary carbonates of the bundle laminaerecord minor, very short-term hydrochemicalchanges in the system. These changes could takeplace over time spans ranging from pluriannual–decennial periods up to several thousand years(Figs 9 and 10) and may be in relation to cyclicalprecipitation shifts.

Infill organization and controlling factorsin basin evolution

A major trend of water input increase and lakelevel rise is clearly recorded by the superpositionof Subunits 1A and 1B, linked to the change fromshallow, saline lacustrine conditions to deeper,freshwater conditions, which persisted until thelake’s final infilling. This change caused the low-order stratigraphic arrangement of the up to 100 mlacustrine infill (Fig. 13, curve 1). Apart from thislow-order, deepening trend, dm- to m-thick cyc-lical sequences also occur in Subunits 1A and 1B.These high-order sequences (Fig. 13, curve 2)resulted from alternating lacustrine flooding andshrinkage episodes, related to changing waterinput/output ratios in the system. The magneto-stratigraphic dating of the lacustrine infill (Huertaet al., 1997; Ferrus, 1998) suggests that thedeposition of the whole low-order sequencelasted around 1Æ5 My and that the periodicityorders of the dm- to m-thick cycles would be closeto an average of 30 ky. The thinnest rhythmicclay–carbonate microlaminations (a few micro-metres to a few millimetres thick) resulted fromvery short-term, probably seasonal changes,



whereas the carbonate–clay rhythmic bundleswould develop during decennial to millennialtime spans (Fig. 13, curves 3 and 4).

The stratigraphic arrangement of the lacustrineAs Pontes Basin records the long-term (i.e.1Æ5 My) trend of expansion, deepening and infil-ling of a lake and the repeated, short-term (i.e.around 30 ky on average) water level oscillationsthat were superimposed on this trend. The causesof these water balance changes can be constrainedby taking into account the palaeotectonic, pala-eogeographic and palaeoclimatic setting of thebasin. Thus, the low-order sequence transitionfrom Subunit 1A to Subunit 1B was related to awater volume increase from about 1 hm3 in theearliest evolutionary stages to 11Æ5 hm3 in themaximum lacustrine spreading and deepeningstage (Fig. 2a¢ and b¢). There is no palaeobotanicalevidence for significant climatic forcing (i.e. arainfall increase) during the transition from 1A to1B (Baltuille et al., 1992). Therefore, this watervolume increase must have been caused by the

coupled influence of: (1) the increment of accom-modation in the basin caused by increasingsubsidence rate related to the synchronous activ-ity of reverse and normal faults in the strike-slipfault system, which outstripped sedimentationrates (Ferrus, 1998); and (2) the enlargement ofthe catchment of the As Pontes eastern subbasin(Barso et al., 2000). Owing to its location on theuplifted zones of the strike-slip system, tectonicscould have influenced drainage evolution in thecatchment area, but simple headward extensionof the drainage network can also account for thereported catchment enlargement.

The warm subtropical climatic regime in NWSpain and its palaeolatitudinal position duringthe Oligocene made this region very sensitive tohigh-frequency cyclical palaeoclimatic variations(i.e. from dry to moister conditions). This influ-ence was significant in terms of water andsediment contributions, palaeohydrochemistryand water stratification. The changes in theseparameters would have resulted in the cyclical

Fig. 13. Evolution of the lacustrine record in the As Pontes Basin. The major tectonic and hydrological changes areindicated. Curve 1, low-order lacustrine basin infill sequence that was controlled by tectonics and the headwardspread of the catchment drainage. Curve 2, cyclically arranged mudstones and carbonates (Subunit 1A) and inter-bedded turbidite beds, rhythmite bundles and marginal bench facies (Subunit 1B), which were mainly controlled byclimatic oscillations. Curve 3, decennial to millennial carbonate–clay rhythmic bundles. Number 4 indicates prob-able seasonal clay–carbonate couplets. See text for further explanation.



dm- to m-thick sequences in both subunits (curve2, Fig. 13) and mainly record this climatic forcingon the lacustrine system. The limited total thick-ness of these cyclical sequences precludes thedefinitive statistical testing of their periodicityand their possible attribution to orbitally forcedperiodical cycles (Berger & Loutre, 1994). Never-theless, their range of thickness and estimatedaverage duration (i.e. 30 ky) are similar to thatobserved in other lacustrine sequences that havebeen interpreted as climatically forced cycles(Olsen, 1986; Anadon et al., 1991).

CONCLUSIONS

Tectono-sedimentary interactions in the Oligo-cene As Pontes Basin resulted in the deposition ofa low-order lacustrine sequence (Unit 1), whichrecords the establishment, spreading, deepeningand final infilling of a small, tectonically forcedlake basin.

The most significant depositional, palaeoenvi-ronmental and palaeohydrological shift identifiedin the As Pontes lacustrine infill resulted from thecoupled influence of tectonic and morphologicalprocesses affecting the basin catchment system.Thus, a tectonically driven increase in accommo-dation space and increasing water input (causedby the headward spread of the watershed)account for the significant deepening and dilu-tion of the lacustrine system and the shifting froma palustrine–lacustrine, shallow ramp-dominatedframework to deeper and more stable subaqueousconditions. In addition to this tectonic–morpho-logical forcing, minor oscillations in the waterlevel also influenced the lacustrine sedimentationand gave rise to higher order cycles. These minoroscillations were probably climatically forcedand modulated the evolution of the lacustrinesystem. Therefore, the lower shallow palustrine–lacustrine facies (Subunit 1A) were depositedunder low and variable water levels (saline andmainly holomictic conditions), which resulted inthe deposition of cyclically arranged terrigenouscarbonate sequences. The lacustrine sequences inSubunit 1B were also formed mainly underchanging but steadier, meromictic, fresh to oli-gosaline conditions, which favoured the cyclicaldeposition of fine-grained turbidite beds andclay–carbonate rhythmite bundles.

The sedimentological and geochemical data (i.e.short-term increases in Mg/Ca ratio, d18O and d13Ccovariance trends in calcite, abrupt d18O varia-tions) correspond well with the characteristics of

small, closed lakes which, given their sensitivityto climatically forced water balance variations,commonly display very short-term hydrochemi-cal trends.

The research community is already aware thatthe sedimentary record of ancient lakes providessome of the most complete, high-resolutionarchives of palaeoclimatic processes. This isbecause climate is often emphasized as a majorcontrol on lacustrine evolution. The presentstudy reinforces the potential of high-resolutionancient lacustrine records to elucidate short-termpalaeoclimatic changes. Nevertheless, the analy-sis of the long-term evolution of the tectonicallyforced As Pontes Basin illustrates the importanceof understanding the style, intensity and timingof tectonic deformation and drainage evolution inorder to distinguish properly between tectonicand climatic forcing of the sedimentary record.

ACKNOWLEDGEMENTS

This research was funded by the Spanish CICYTprojects AMB92-311, PB94-0826 and PB97-0882-C03-01. Financial support was also provided byENDESA – Fundacio Bosch i Gimpera (projects207 and 2149). The Grup de Recerca de QualitatConsolidat of Geodinamics i Analisi de Conques(2001SGR 00074) also supported this research.We thank A. de las Heras for her helpfulco-operation in the sampling and analysis of thecarbonate samples. P. Anadon, C. Taberner andJ. J. Pueyo are gratefully acknowledged fordiscussions that helped to clarify and improvean earlier version of this paper. The authorsare also indebted to M. Talbot, B. Valero-Garcesand P. Mozley for their critical comments andsuggestions.

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Manuscript received 25 July 2000;revision accepted 23 May 2002.



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